It has recently been discovered that certain pyranone compounds inhibit retroviral protease and thus they are useful for treating patients infected with human immunodeficiency virus (HIV) which results in acquired immunodeficiency syndrome (AIDS). In particular, the pyranone compound of formula I has been found to be especially effective as an inhibitor of retroviral protease. ##STR1##
However, like many other HIV protease inhibitors, these compounds are characteristically lipophilic and thus poorly water soluble. For example, the compound of formula I has an aqueous solubility about 1 .mu.g/ml in the buffer of pH 6.5 (close to the pH of the intestine), which is considered as extremely poor aqueous solubility and would be expected to provide very low oral bioavailability in the free acid form. It is well known that an active drug substance or therapeutic moiety administered by any route must possess some aqueous solubility for systemic absorption and therapeutic response. Poorly water soluble compounds often exhibit either incomplete or erratic absorption and thus produce a minimal response at desired dosage.
Attempts were made to identify salts of the pyranone compounds in solid forms which could improve aqueous solubility. An overriding defect which has however remained is that the formulations in the form of salt are prone to precipitation of the parent free acid in the gastrointestinal tract and hence are not capable to provide a dosage in the desired high concentration to permit convenient use and yet meet the required criteria in terms of bioavailability.
Recognizing the problems, the present invention is directed toward pharmaceutical compositions in a form of self-emulsifying formulations which provide high concentration and high oral bioavailability for pyranone compounds. In particular it has been discovered that the compositions of the present invention allow the preparation of self-emulsifying formulations containing a pyranone inhibitor of retroviral protease in an exceedingly high concentration up to about 400 mg/g to permit convenient oral administration while at the same time achieving improved bioavailability, which is at least two fold higher than the aqueous suspension of the free acid.